You can use them over textures.  Some shaders add things other than just colors too....
If I were you I'd sit down and play with a prop or model and the shaders you have installed...just to see what they do and it would be a learning experience in the surfaces tool also...to see how you can change things in there.
I'm sure someone else will be in here to give you other suggestions and some good ideas  too.  I'm just learning about shaders but that's what I do...practice and experiment.  :biggrin:

 

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